1. Field of the Invention
The present invention relates to an electrode for a non-aqueous secondary battery in which an alloy containing metal that is alloyed with Li is formed on a collector as a plurality of convex portions with gaps disposed thereamong, and a non-aqueous secondary battery using the electrode as a negative electrode.
2. Description of the Related Art
Conventionally, as a negative electrode for a non-aqueous secondary battery such as a lithium secondary battery, a graphite-based electrode containing a graphite-based material as a negative active material has been used. Recently, in order to increase a capacity, in place of the graphite-based electrode, a so-called thin film electrode is proposed, in which a thin film made of metal such as Si, Ge, Sn, or the like that is alloyed with Li or an alloy of the metals is formed on a collector made of a material that is not alloyed with Li, and such a thin film electrode is receiving attention (see JP2001-68094A and JP2001-256968A).
Among these prior art references, JP2001-68094A discloses that a Sn thin film is formed on a copper plate that is a collector by electroplating, and JP2001-256968A discloses that a thin film made of Sn, Zn, Sb, or an alloy containing them is formed on a copper foil by electroplating.
Among the above-mentioned metals, in particular, Sn has a higher energy density (994 mAh/g) compared with a conventional graphite-based negative material, so that Sn is considered as a promising next-generation negative material. However, in an actual electrode, when Li is absorbed electrochemically up to x=4.4 in a composition formula: LixSn, the volume of a thin film constituting an active material expands by 3 to 4 times. Such a volume expansion leads to a decrease in cycle characteristics of a non-aqueous secondary battery, and Sn has a catalytic function, so that it will decompose an electrolyte solution.
In order to solve the above-mentioned problem, an electrode having an intermetallic compound of Sn is proposed (see J. Power Source, 107(2002), pp. 48-55)
In this document, a Sn thin film is formed on a collector made of a Cu foil by electroplating, and heat-treated at a temperature in the vicinity of the melting point of Sn, whereby a Sn-based metal thin film with a gradient structure in which each element interdiffuses at a Cu—Sn interface is obtained. Cu6Sn5 formed in this case is capable of repeatedly absorbing/desorbing Li, is less changed in volume compared with Sn, and has no catalytic function. Thus, Cu6Sn5 is expected as a negative material that can solve the problem peculiar to the Sn thin film.
Furthermore, as a result of the Cu—Sn interdiffusion involved in heat treatment, the collector and the Cu6Sn5 alloy thin film as the active material layer are integrated more closely. Therefore, even when the expansion/contraction of the active material involved in a charging/discharging cycle occur, the active material is unlikely to drop from the collector, and cycle characteristics are enhanced compared with the case of using Sn alone.
However, due to that fact that the Sn thin film obtained by electroplating is fine and continuous, and that the density of the Sn thin film is further increased by the subsequent heat treatment, as the thickness of the active material layer is increased, the electrode including the collector is influenced by the volume expansion of the active material layer involved in absorption/desorption of Li. Consequently, the generation of cracks in the active material layer, the excess expansion of the electrode, the wrinkling of the collector, and the like are exhibited remarkably, and a capacity and cycle characteristics are decreased.